fix: handle discovery sessions in UnBindISCSISession

Discovery sessions have nil Target and nil ITNexus. The cleanup
path crashed with nil pointer dereference when trying to remove
ITNexus or access Target.Sessions for discovery sessions.

Guard against nil Target (just release TSIH) and nil ITNexus.
Also add nil safety to clearHostIP helper.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/port/iscsit/iscsid.go

@@ -376,12 +376,19 @@ func (s *ISCSITargetDriver) handler(events byte, conn *iscsiConnection) {
 }
 
 func (s *ISCSITargetDriver) clearHostIP(conn *iscsiConnection) {
+	if conn.conn == nil {
+		return
+	}
 	IPMutex.Lock()
-	remoteIP := strings.Split(conn.conn.RemoteAddr().String(), ":")[0]
+	defer IPMutex.Unlock()
+	addr := conn.conn.RemoteAddr()
+	if addr == nil {
+		return
+	}
+	remoteIP := strings.Split(addr.String(), ":")[0]
 	if CurrentHostIP == remoteIP {
 		CurrentHostIP = ""
 	}
-	IPMutex.Unlock()
 }
 
 func (s *ISCSITargetDriver) rxHandler(conn *iscsiConnection) {